1. Field of the Invention
The present invention relates to a curable composition that is suitably used for an ink composition, a coating material, an adhesive and the like. The invention also relates to an ink composition, which includes the curable composition, suitably used for an inkjet-recording method. The present invention further relates to a planographic printing plate obtained by using the ink composition.
2. Description of the Related Art
Curable compositions that cure at high sensitivity by irradiation of active radiation and form a cured film have been used in various applications, and are attracting attention as the principal component of the ink composition used in the image-recording method described below.
Cyclic ether compounds, such as 3- or 4-membered rings, are known as materials that exhibit high reactivity, and have been used as a polymerizable compound that are included in curable compositions applied to thermal polymerization using photocationic polymerization or acid anhydrides (see, for example, Japanese Patent Application Laid-Open(JP-A) Nos. 11-43540 and 11-60702).
As an image recording method of forming an image on a recording medium, such as a paper or the like, on the basis of image data signals, there are an electrophotographic method, a sublimate-type and melt-type thermal transfer method, and an inkjet method. Among these, the inkjet method is feasible with a low cost device, in which an image is directly formed on a recording medium by discharging ink on a necessary image area only. Therefore, the ink can be efficiently used and the running cost is low. In addition, the inkjet method is superior as an image recording method since noise is low.
According to the inkjet method, it is possible to print not only on a normal paper but also on a non-absorptive recording medium, such as a plastic sheet and a metal plate. However, in this case, speeding-up upon printing and high definition are key matters. Further, there is a property in which the time required for drying and curing droplets after printing have a large effect on productivity of printed materials or sharpness of printed images.
As one of the inkjet methods, there is a recording method that uses inkjet recording ink curable by irradiation of radiation. According to this method, since ink droplets are cured by irradiating radiation immediately after or after a predetermined time after ink is ejected, the productivity of printing can be increased and sharp images can be formed.
When high sensitivity of inkjet recording ink curable by irradiation of radiation, such as ultraviolet rays, is realized, high curability with respect to radiation is given, which causes numerous benefits, such as the improvement of the productivity of inkjet recording, the reduction in power consumption, the extension of lifetime due to a reduction in load on a radiation generator, and the prevention of the occurrence of volatilization of low molecule substances by insufficient curing. Further, high sensitivity means the improvement of the strength of an image formed by, in particular, inkjet recording ink. Particularly, if it is applied to the formation of a planographic printing plate, strength of the cured image area is increased, and thus high printing durability can be obtained.
Recently, such an inkjet method curable by radiation, for example, ultraviolet rays has been drawing attention from the point of relatively less odor, quick drying properties, and a capability of recording onto a non-ink absorptive recording medium. Ultraviolet ray curable ink compositions for inkjet using radical polymerization are disclosed (for example, see JP-A No. 63-235382, JP-A No. 3-216379, JP-A No. 5-214280, JP-B No. 6-21256, and JP-B No. 6-62905).
Further, with an object of providing inkjet recording ink capable of recording an image without bleeding, but with high sensitivity, and high adhesiveness onto a recording medium, even for a substrate that is normally difficult to directly record by an inkjet recording method, and highly safe due to less irritation and sensitization to skin, compositions including colorants and polymerizable compounds including a specific radial polymerizable acrylate compound group are proposed (for example, see JP-A No. 2003-192943 and JP-A No. 2003-192944). These radical polymerizable inks are superior in a curing rate, and are capable of forming an image without bleeding, but have a problem in that the adhesiveness onto a recording medium is decreased due to volume shrinkage upon curing.
For this reason, with an object of improving the adhesiveness onto a recording medium, cationic polymerizable ink compositions with a lower shrinkage rate upon curing by ultraviolet rays are proposed (see, for example, JP-A No. 9-183928). However, cationic polymerizable ink has a big disadvantage for practical use in that the stability during the storage is not sufficient due to a reaction based on acid generated over time. Therefore, as an attempt to improve the storage stability, techniques of adding a basic compound or a thermal base generator are proposed (for example, see JP-A No. 2003-312121, JP-A No. 2003-341217, and JP-A No. 2004-91558). However, it was found a new problem occurs in that the basic compound inhibits the function of acid generated by exposure, thus decreasing the ink curing sensitivity.
Further, multifunctional cyclic ethers used for improving the sensitivity of cationic polymerizable ink give a polymer, which is obtained by curing the multifunctional cyclic ethers, having a highly crosslinked three-dimensional structure. For this reason, the cationic polymerizable ink including such multifunctional cyclic ethers has disadvantages such as deterioration of the flexibility of the film after ink curing, easier cracking, exfoliation, and decreased durability of the cured film on recording medium.
Currently, as described above, there is still no curable composition that can be applied to UV-curing ink composition and the like, which has highly sensitivity to radiation irradiation, a high-strength film formability, and superior in flexibility of cured materials.